Franklin: Getting the `hljs` syntax highlighting to work on Pluto codeblocks

I think the root cause is that when the output is generated, it's already processed as HTML so Franklin doesn't have the chance to apply the hljs formatting.

This is coming up in the context of another [attempt](https://github.com/JuliaActuary/JuliaActuary.org/pull/28#issuecomment-1237342542) at using PlutoStaticHTML on the JuliaActuary website. For example, this [notebook output](https://thirsty-davinci-e0b972.netlify.app/previews/pr28/notebooks/mortalitytablesdataframe/) doesn't have the same code formatting as the codeblocks on this [Franklin-markdown-based page](https://thirsty-davinci-e0b972.netlify.app/previews/pr28/tutorials/survivalfitting/)

I think by writing the Pluto code to a simple markdown block instead of an HTML block then Franklin/`hljs` would get a chance to process the syntax highlighting. Is there an existing/easy way to do this?